The present invention relates to valve diagnostics, and more particularly, to the non-intrusive measurement of valve stem displacement during valve actuation.
Diagnostic techniques for determining the operating characteristics of valves in the field have been in general use for a number of years. These techniques usually involve connecting sensors of various types to portions of the valve system whereby upon actuation of the valve, a record of the relationship between the independent actuating variable and the dependent response variable can be obtained and analyzed. The response variable can be force-related or displacement-related, depending on the type of valve and the accessibility of the components in the transmission mechanisms in the valve. Desirably, the response variable is sensed as close as possible to the valve seat, thereby obtaining a more reliable indication of the actual thrust of the valve member against the seat for a given degree of actuation.
U.S. patent application Ser. No. 610,450 filed Nov. 8, 1990 (now U.S. Pat. No. 5,056,046) as a Continuation-In-Part of U.S. patent application Ser. No. 368,604 filed Jun. 20, 1989 (now abandoned) discloses a non-intrusive technique for measuring the stem displacement of a valve of the type in which a portion of the stem is visible. The preferred embodiment of the sensor is sensitive to light, especially in the visible spectrum whereby discrete signals generated in a video camera due to movement of the stem, are converted to distance units for measuring the stem displacement. This technique is easy to implement in the harsh environment of a nuclear power plant, and provides sufficient resolution for achieving improved diagnostics relative to previously available techniques.
The need for even more easily implemented, more precise techniques continues to grow, however, and the present invention represents such improvement.